David Karp in the Flatiron District offices of his blogging site, Tumblr, which Yahoo! bought for $1.1 billion. Photo: AFP/Getty Images

David sporting his trademark moppy due at age 8. (
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David sitting with mom Barbara. (
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Shaggy-haired city hipster and Tumblr founder David Karp yesterday had two words for the $1.1 billion cash sale of his popular blogging site to Yahoo!

“F–k yeah.’’

The 26-year-old wunderkind, who grew up on the Upper West Side strolling the Museum of Natural History and sketching dinosaur bones, posted a message — where else? — on Tumblr to calm his users’ fears of a tech takeover at the Flatiron District-based blogging site.

“Before touching on how awesome this is, let me try to allay any concerns: We’re not turning purple. Our headquarters isn’t moving. Our team isn’t changing,’’ wrote Karp, known for his trademark red plaid shirts, gray hoodies and sneakers.

Simple, direct and, above all, focused, Karp was described by his parents and friends yesterday as a humble genius who has kept his eye on the ball since he dropped out of the prestigious Bronx HS of Science after his first year to concentrate on his “passion” — computers.

“If you knew what an exceptional kid he was, you knew something big would happen. He is so brilliant,” Karp’s mom, Barbara, 62, told The Post at her Upper West Side home.

She said that when her son inked the deal — the largest for any social-media company in the city — Saturday, “He sent me a text, ‘Mom, can I come bug you?’

“He immediately came over,” she said. “It was very emotional. . . . We both got teary-eyed” and there were “lots of hugs.

“This is a big deal — Tumblr has been his baby for many years,” said the mom, who had let her son use the bedroom in her apartment to launch his Web platform in 2007.

Tumblr now boasts 100 million users — including Lady Gaga — and 90 million posts a day.

Barbara Karp said her CEO son wants to do good with the dough he’ll get from the sale, estimated at $200 million to $250 million.

“He said, ‘Mom, I’m now at that point I can be considered a philanthropist,’ ” she said. “He can do something really good now.’’

The mom was a science teacher at the private Calhoun School on the Upper West Side when the family decided to let David, the oldest of two sons, leave Bronx Science at age 15.

David got tutors in English, math and science but spent his free time learning Japanese and taking art classes at the museum.

“He’s a New York kid,” Michael Karp, 61, said.

David zeroed in on programming. He had already taught himself how to code in the Web language HTML — at age 11.

“I remember getting him his first computer, an Apple. That was even before the Internet,’’ said his father, a composer who wrote the Yankee theme for the team’s YES network.

“I remember going over it with him, figuring out what dial-up was, and he really loved it.

“I’d bring back books from The Strand on different kinds of computer things, and he worked on his own.”

The skinny kid with the big smile was soon landing jobs at Web startups.

“I remember when he came to me, and he told me he was going to do this thing. He said he was going to call it Tumblr,’’ his dad recalled.

“I had no idea what Tumblr was at that point.”

As Tumblr took off, David Karp moved with his girlfriend, Rachel Eakley, 29, to a $1.6 million loft in Williamsburg, Brooklyn.

He and Eakley, a chef and grad student, have a bulldog, Clark.

Karp has several motorcycles, including one with a side car, and a Vespa scooter. He’s currently rebuilding an old Honda in his living room, his mom said.

If it were up to Karp, that’s probably all he’d have there.

He has said he doesn’t have any books or clothes — apparently an attempt to emulate the austerity of his idol, Steve Jobs.

“He would watch [Jobs’] stockholder speeches each year. He would just love his sense of design, sense of purpose, sense of simplicity,” Michael Karp said.

Chris Cunningham, founder and CEO of the Web advertising firm appssavvy, told The Post that Karp was a “cool cat.”

“He has that moppy-haired look like a guy you’d pass in the West Village or East Village,” he said. “He’s very eccentric, charismatic and very intelligent and unusual . . . And, yeah, he was born and raised in New York.”

At a press event with Yahoo! at the Renaissance Hotel in Times Square last night, Karp beamed in a hoodie and sneakers as Mayor Bloomberg hailed the sale. Karp’s mom was by his side.

Karp, who will still stay on to run the site, later told The Post: “Tumblr absolutely started here, but the whole network, if you look at it, has a lot of the same characteristics as the city, a lot of the crossover.

“It’s not about one industry; it’s about a whole diverse set of industries, living side be side and occasionally overlapping in really interesting ways.”